High-Order Multilayer Input-Absorptive RF Filter With Wideband Quasi-Flat Group Delay and Multiple Stopband Transmission Zeros

This paper presents a high-order multilayer RF filter with wideband quasi-flat group-delay response and sharp multi-transmission-zero (TZ) stopbands. It is composed of three in-series-cascaded stages of low-order/single-unit input-absorptive wideband bandpass filters (BPFs). Its constituent BPF unit, which is based on a modified two-layer microstrip-to-microstrip vertical transition and a shunt resistivelyterminated microstrip line and exhibits quasi-flat group-delay response and broadband input-absorptive capabilities, is firstly discussed. Due to the employed short-circuit-ended two-section microstrip stub in the modified vertical transition, two close-to-passband TZs are realized to obtain a sharper-rejection BPF response. To achieve highly-attenuated multi-TZ stopbands, a high-order wideband quasi-flat-group-delay BPF with three in-series-cascaded single-unit BPFs using different impedanceparameter values for the short-circuit-ended microstrip stubs is then studied. For experimental-validation purposes, a 2-GHz proof-of-concept microstrip prototype in a two-layer substrate is simulated, built, and tested. The measured input-absorptive BPF is centered at 1.956 GHz with 3-dB absolute bandwidth of 0.828 GHz and maximum group-delay variation of ± 0.05 ns from 1.387 to 2.618 GHz (i.e., 1.231-GHz absolute bandwidth).